(a) Field of the Invention
The present invention relates to a cement admixture, and more particularly to a cement admixture for forming concrete with good strength and which improves dispersibility of cement particles, increases composition fluidity even at a high range water-reducing formulation, prevents a decrease of fluidity over time, and offers good workability by supplying an adequate amount of air flow continuously.
(b) Description of the Related Art
Cement compositions, such as concrete or mortar, are hydraulic substances that are hardened by reaction of cement and water. Physical properties of cement compositions, such as compressive strength after hardening, vary depending on the amount of water used. In general, while an increase in the amount of water used improves workability, it decreases compressive strength and causes cracking. Therefore, the amount of water used in cement compositions is limited. The Korean Industrial Standard (KS) F 2560 recommends use of a chemical admixture for concrete to reduce the amount of water used.
Chemical admixtures are classified as AE (air-entraining) admixtures, water-reducing admixtures, and high range water-reducing admixtures. The AE admixture is a chemical admixture comprised in a cement composition to increase the amount of small air bubbles, and it is mixed with a water-reducing admixture or a high range water-reducing admixture to form an AE water-reducing admixture (air entraining and water-reducing admixture) and a high range AE water-reducing admixture. When an AE water-reducing admixture is used, the amount of water used can be decreased by about 10 wt %, and a high range AE water-reducing admixture can reduce the amount of water used by more than 18 wt %.
The water-reducing admixture is made of a lignin compound, and the high range water-reducing admixture is made of a naphthalene-formalin condensate and a melamine condensate.
Because a slump loss of concrete in a cement composition occurs within 30 minutes, all work from concrete mixing to casting must be done in a short time. With the recent increase in unit content of water due to a change of aggregates, use of mechanized modern equipment, and an increase in traffic volume, a new chemical admixture having superior slump retention characteristics while having better water-reducing properties than the conventional AE water-reducing admixtures is required. Accordingly, admixture makers are actively developing high range AE water-reducing admixtures that have good water-reducing properties and low slump loss, and that can be added to the concrete composition at the concrete plant.
Currently, high range AE water-reducing admixtures, such as naphthalene sulfonic acid formaldehyde condensate salt (naphthalene based), melamine sulfonic acid formaldehyde condensate salt (melamine based), and Polycarboxylate(poly carboxylic acid based), are on the market. However, these high range water-reducing admixtures have some problems. For example, the naphthalene based and melamine based high range water-reducing admixtures have a problem of fluidity decrease (slump loss) while they have superior hardening characteristics. On the other hand, the biggest problem of the polycarboxylic acid based high range water-reducing admixtures is hardening retardation. However, with the recent development of a polycarboxylic acid based concrete admixture having superior fluidity, it is possible to obtain good fluidity by adding only a small amount, and the problem of hardening retardation is solved.
As a polycarboxylic acid based high range AE water-reducing admixture, Japan Patent Publication No. Sho 59-18338 discloses a copolymer prepared from a polyalkylene glycol mono(meth)acrylic acid ester monomer, a (meth)acrylic acid monomer, and a monomer that can be copolymerized with them. Additionally, Japan Patent Publication No. Hei 5-238795 discloses a copolymer prepared from a polyalkylene glycol diester monomer having an unsaturated double bond, a monomer having a dissociated group, and another specific monomer. While these polycarboxylic acid based high range AE water-reducing admixtures have good water-reducing properties and slump retention characteristics, there is much room for improvement.
Japan Patent Publication No. Sho 57-118058, Japan Patent Publication No. Hei 8-283350, Japan Patent Publication No. Hei 9-142905, and so forth disclose cement dispersing agents comprising a copolymer prepared from a polyethylene glycol monoallyl ether monomer and a maleic acid monomer. However, these cement dispersing agents do not have satisfactory dispersibility, especially at a high water-reducing formulations, because the polyethylene glycol monoallyl ether monomer and the maleic acid monomer are not easily copolymerized.
Japan Patent Publication No. Hei 10-194808 discloses a cement dispersing agent comprising a copolymer prepared from a polypropylene glycol polyethylene glycol mono(meth)allyl ether monomer and an unsaturated carboxylic acid monomer. Because this cement dispersing agent has a high hydrophobic polypropylene glycol content, it does not have sufficient dispersibility. Therefore, it should be added in a large amount to obtain sufficient dispersibility, and it is impossible to obtain sufficient dispersibility at a high water-reducing formulation.